Bug 1618897 [wpt PR 22021] - [CSP] Factorize SVGElement & MHTMLElemen…

…t nonce hiding., a=testonly

Automatic update from web-platform-tests
[CSP] Factorize SVGElement & MHTMLElement nonce hiding.

According to:
whatwg/html#2373
html and svg Element are hiding their nonce when there are at least one
Content-Security-Policy defined from an HTTP header.

The two implementation:
- HTMLElement::InsertedInto
- SVGElement::InsertedInto

were hidding the nonce slightly differently. To prevent further
divergence, factorize this implementation into Element::HideNonce() and
call it from both places.

Bug: 1053496
Change-Id: I3cbad88f70c61591bef060d4188c82388e6001d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078536
Commit-Queue: Arthur Sonzogni <arthursonzognichromium.org>
Reviewed-by: Mike West <mkwstchromium.org>
Reviewed-by: Fredrik Söderquist <fsopera.com>
Cr-Commit-Position: refs/heads/master{#746837}

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wpt-commits: 06705ea82c8a9d1866665c8abd069dd3b0f8c12b
wpt-pr: 22021

UltraBlame original commit: 24ff6c7777429ca3691d2c2d66c9a2d09651f701